The invention relates to a rectifying column for the extractive distillation of close-boiling or azeotropic boiling mixtures, comprising
a column main section and a raffinate section located above said column main section;
an evaporating device located at the lower end of the column and having at least one evaporator;
an arrangement on the cap of the column with a raffinate outlet, a condenser and a device for partially recycling the raffinate liquefied in the condenser;
an inlet located between the main section of the column and the raffinate section for charging a starting mixture to be separated; and
a solvent inlet located at the top side of the raffinate section for feeding an extracting agent.
In addition to the column described above, known installations for extractive distillation have a second rectifying column in which the sump product withdrawn from the first column is separated into pure extracting agent and a pure product current (ULLMANN""s Encyclopedia of Technical Chemistry, volume 2, 4th edition, page 511; EP-B 0 216 991). The extracting agent is collected in the sump of the second column and is recycled into the solvent inlet of the first column. A pure product current, hereinafter also referred to as the extract, is withdrawn at the cap of the second column. The second column required within the framework of the known measures for recovering the extracting agent has a column section located above the inlet, said section being designed as an updraft part with a plurality of theoretical separating trays; a column xe2x80x9cshootxe2x80x9d designed as the updraft part located below the inlet; a sump heating system; as well as an arrangement on the cap of the column comprising a product outlet, a condenser and a device for partially recycling the product liquefied in the condenser.
The space required for installing a distilling plant with two columns including the pipeline system associated therewith upstream is quite considerable and is in some cases not available when such a distilling apparatus needs to be installed in an existing chemical plant.
A rectifying column for extractive distilling is known from DE-A-33 27 952 which permits separating an azeotropic two-substance mixture into its individual components. Said rectifying column has a column main section with two chambers connected in parallel, whereby one chamber is open at the top and bottom sides and can be designed as the stripping part for separating the raffinate from the mixture containing the extracting agent. The other chamber is closed against the interior space of the column at the top side and open only at the bottom side. A product that is substantially free of extracting agent can be withdrawn from said chamber via a side outlet. Furthermore, the sump of the column at the bottom end of the column is connected with the solvent-feeding inlet by a device for recycling the extracting agent.
The invention is based on the problem of further developing a rectifying column with the structure known from DE-A-33 27 952 in such a way that the amounts of stripping steam required for the separation processes can be admitted into both chambers, and that the extracting agent can be recycled from the sump of the column into the inlet of the solvent with high purity.
The object of the invention and the solution of the problem is a rectifying column for the extractive distillation of close-boiling or azeotropic boiling mixtures, comprising
a column main section having two chambers connected in parallel;
a raffinate section located on top of the column main section;
an evaporating device located at the lower end of the column and having at least one separator;
an arrangement at the cap of the column comprising a raffinate outlet, a condenser and a device for partly recycling the raffinate liquefied in the condenser;
an inlet located between the column main section and the raffinate section for feeding a starting mixture to be separated; and
a solvent inlet at the top side of the raffinate section for feeding an extracting agent;
whereby one chamber of the column main section is open at the top and bottom sides, contains installations for improving the substance exchange, and is designed as a stripping section with a plurality of theoretical separation stages for separating the raffinate from the mixture containing the extracting agent; whereby the other chamber of closed at the top side versus the interior of the column and is open at the bottom side, contains installations for promoting the material exchange, and, in a space located above said installations, has devices for withdrawing a substantially extracting agent-free, gaseous product as well as for refluxing a liquefied partial product stream; and whereby the sump of the column is connected at the lower end of the column with the solvent inlet by a device for recycling extracting agent; and whereby, furthermore, the following features are realized according to the invention:
a column stripping section with a plurality of theoretical separation stages is arranged between the sump of the column and the column main section, with the extracting agent being concentrated in said stripping section from the top down;
a catching tray is arranged below the column main section, said catching tray being permeable to gas and having its outlet for liquid connected to a continuous-flow flow heater for heating the liquid draining from the two chambers of the column main section;
the heated mixture exiting from the continuous-flow heater can be fed below the catching tray into the stripping section of the column.
According to a preferred embodiment of the invention, the continuous-flow heater is heated by heat exchange with the stream of extracting agent withdrawn from the sump of the column.
The instruction according to the invention reduces the plant for extractive distilling to one single column, where sections for the recovery of the extracting agent as well as for concentrating the extract are integrated. The extracting agent is charged above the raffinate section. A portion of the components of the starting mixture preferably dissolves as extract and is washed out from the stream of extracting agent in the raffinate section. The other portion of the starting mixture passes as gaseous raffinate to the cap of the column. In the chamber of the column main section that is open at the top and bottom sides, the dissolved raffinate component is concentrated by vapor stripping. The mixture exiting on the bottom side of said chamber substantially consists of the extracting agent and the extract dissolved therein and still only traces of the dissolved raffinate in most cases. In the column stripping section that is located below said chamber, the extract is stripped from the extracting agent by vapor stripping. A partial stream of the vapors rising in the column enters the chamber of the column main section that is open at the top and bottom sides and strips the raffinate in said chamber. Another part stream enters the second chamber of the column main section that is closed against the interior space of the column. In said chamber, which is referred to in the following also as the product section, the concentration of the extracting agent that rises together with the product vapors according to the solvent partial pressure, is reduced by product reflux and passed with the product reflux into the column stripping section. In the sump of the column, the extracting agent is collected substantially in the pure form. It is cooled and recycled into the solvent inlet above the raffinate section.
It is within the scope of the invention that the column main section consists of two independent column passages which are substantially arranged next to each other and connected at their lower ends with the column stripping section by a distributor. However, the column main section is preferably designed in the form of a cylindrical column passage that is installed between a column section forming the raffinate section, and the column stripping section, and which contains an insert forming the chamber that is closed at the top side. The insert can have a cylindrical shape and it can be arranged in the center. However, a preferred solution that is simpler in terms of plant engineering is that the insert consists of a partition that is connected with the jacket of the column passage and extends in the longitudinal direction of the column, and a cover that is connected with the partition and a jacket segment of the column passage. The partition divides the cross sections of the column passage in two sections. The division of the sections is dependent upon how the rising vapors have to be divided and allocated to the two chambers.
Between the raffinate section and the column main section, provision is usefully made for a catching tray for liquid, said tray being permeable to gas, whereby a liquid distributor is connected with the catching tray. Said distributor feeds the draining liquid to the installations in the chamber of the column main section that is open at the top side.
Above the raffinate zone, the rectifying column as defined by the invention may have a column section for raffinate stripping, said section extending from the solvent inlet up to the cap of the column and usually having a plurality of theoretical separation stages.
The rectifying column as defined by the invention permits extractive distillation within a confined space. This is advantageous, for example when additional apparatus for extractive distilling has to be installed in an existing chemical plant for expanding the production capacity. It has been surprisingly found that the rectifying column as defined by the invention not only requires substantially less space than a distilling plant with two columns according to the prior art, but also distinctly reduces the energy requirements without any change in the through-put capacity, product purity and product yield.